Elastohydrodynamic lubrication (EHL) is the mode of lubrication that exists in non-conforming concentrated contacts. Examples include the contact between meshing gear teeth used in hypoid axles, worm gears, etc. and between the components in a rolling element bearing. In these contacts the load is supported over a very small contact area which results in very high contact pressures. As lubricants are drawn into the contact zone by the movement of the component surfaces, the lubricant experiences an increase in pressure. Pressures on the order of 1 GPa and above are common in EHL contacts. Most lubricating oils exhibit a large increase in viscosity in response to higher pressures. It is this characteristic that results in the separation of the two surfaces in the contact zone.
If there is relative sliding between the two contacting surfaces in the central contact region, the lubricant is sheared under these high-pressure conditions. The shearing losses depend on how the oil behaves under these extreme conditions. The properties of the oil under high pressure, in turn, depend on the type of base stocks used in the manufacture of the finished lubricant. The generation of the EHL film is governed by what happens in the inlet region of the contact; however, the energy losses are governed by what happens when the lubricant is sheared in the high-pressure central contact region.
The resistance of the lubricant to the shearing effects within an EHL contact is referred to as traction. This is not to be confused with friction, which is associated with surface interactions. The traction response is dominated by the shear behavior of the lubricant in the central high contact pressure region of an EHL contact. The traction properties generally depend on the base stock type.
Traction coefficients can be defined as the traction force divided by the normal force. The traction force is the force transmitted across a sheared EHL film. The normal force or contact load is the force of one element (such as a roller) pushing down on a second element. Therefore, the traction coefficient is a non-dimensional measure of the shear resistance imparted by a lubricant under EHL conditions. Lower traction coefficients result in lower shearing forces and hence less energy loss if the two surfaces are in relative motion. Low traction is believed to be related to improved fuel economy, increased energy efficiency, reduced operating temperatures, and improved durability.
FIG. 1 compares traction curves for a typical mineral oil and a typical PAO. As two surfaces move past one another, if they are moving at the same speed, there is pure rolling and no sliding. The lubricant is not sheared in the contact zone and no traction force is generated (% slide-roll ratio=0; traction coefficient=0; see FIG. 1). The % slide-to-roll ratio is defined as the difference in speed of the two surfaces divided by their average speed and multiplied by 100%. As the ratio of sliding to rolling increases (i.e., moving along the curves in FIG. 1 to the right) the lubricant begins to be sheared between the two surfaces, and since the oil is also under very high pressure, there is a rapid rise in the traction force which is transmitted across the lubricant film. In some cases, the lubricant behaves like an elastic solid. As the sliding increases still further, the traction coefficient may reach a maximum beyond which there is no further significant increase in traction. Under the conditions that exist in many gear and bearing contacts, this maximum is thought to be associated with reaching a maximum yield stress that can be supported by the lubricant. This maximum is determined by the conditions in the contact as well as the type of lubricant used.
As shown in FIG. 1, the PAO has a much lower traction coefficient, relative to mineral oil, over the range of slide-roll ratios, pressures and temperatures evaluated. This means that less energy will be required to shear the EHL film which separates moving surfaces. When gear oils are formulated based on PAO vs. mineral oil, one sees the same lowering of the traction coefficient. This concept is well documented in the industry.
It is also well documented that certain types of synthetic base stocks can provide reduced traction over a wide range of conditions. FIG. 2 is a qualitative comparison of traction coefficients of typical mineral oils, PAOs, and polyalkylene glycols (PAGs).
U.S. Pat. No. 4,956,122 discloses combinations of high and low viscosity synthetic hydrocarbons. A composition is claimed comprising a PAO having a viscosity of between 40 and 1000 cSt (100° C.), optionally further comprising a synthetic hydrocarbon having a viscosity of between 1 and 10 cSt (100° C.), a carboxylic acid ester having a viscosity of between 1 and 10 cSt (100° C.), an additive package, and mixtures thereof.
U.S. Pat. No. 5,360,562 teaches a transmission fluid comprising a PAO having a viscosity of from about 2 to about 10 cSt (100° C.) and a PAO having a viscosity in the range of about 40 to about 120 cSt (100° C.) and devoid of high molecular weight viscosity index improvers.
U.S. Pat. No. 5,863,873 teaches a composition comprising a base oil having a viscosity of about 2.5 to about 9 cSt (or mm2/s) at 100° C. as a major component and a fuel economy improving additive comprising a polar compound with a viscosity greater than the bulk lubricant present from 2 to about 15 wt % of the composition. The compositions are said to improve fuel economy in an internal combustion engine.
U.S. Pat. No. 6,713,438 is directed to engine oils comprising a basestock having a viscosity of from 1.5 to 12 cSt (100° C.) blended with two dissolved polymer components of differing molecular weights.
U.S. Pat. No. 6,713,439 is directed to a composition comprising a PAO with a viscosity of about 40 cSt (100° C.), a basestock having a viscosity of from 2 to 10 cSt (100° C.), and a polyol ester.
Publication WO 03/091369 discloses lubricating compositions comprising a high viscosity fluid blended with a lower viscosity fluid, wherein the final blend has a viscosity index greater than or equal to 175. In an embodiment, the high viscosity fluid is preferably a polyalphaolefin and/or the lower viscosity fluid comprises a synthetic hydrocarbon. In another embodiment, the novel lubricating compositions of the present invention further comprise one or more of an ester, mineral oil and/or hydroprocessed mineral oil.
Publication US2003/0207775 is directed to compositions including a higher viscosity fluid (40 cSt to 3000 cSt at 100° C.) and a lower viscosity fluid (less than or equal to 40 cSt at 100° C.) wherein the final blend has a viscosity index of greater than or equal to 175. All of the examples include a PAO 2 (“SHF™ 23”) as well as a higher viscosity PAO.
Publications US 2004/0094453 and 2005/0241990 are directed to the use of Fischer-Tropsch derived distillate fractions, the latter patent application said to be related to low traction coefficients.
Publication US2004/029407 discloses lubricating compositions comprising high viscosity PAOs blended with a lower viscosity ester, wherein the final blend has a viscosity index greater than or equal to 200, including a composition comprising a PAO having a viscosity of greater than or equal to about 40 cSt at 100° C. and less than or equal to about 1,000 cSt at 100° C.; and an ester having a viscosity of less than or equal to about 2.0 cSt at 100° C., wherein said blend has a viscosity index greater than or equal to about 200.
“Effect of Lubricant Traction on Scuffing”, STLE Tribology Transactions, Vol. 37 No., Apr. 2, 1994, p. 387-395 reported the use of low traction PAO-based lubricants with mineral oils in basestock, antiwear and extreme pressure (EP) formulations and at both high (greater than 6) and moderate (approximately 1.2) specific film thickness lambda. At lambda greater than 6, the benefits of the synthetics over their mineral counterparts ranged from 25 percent to 220 percent and at lambda nearly 1.2, the benefits were a uniform 40 percent. It was particularly interesting to observe that the antiwear PAO-based oil gave a similar scuff load per unit contact width to an EP mineral gear oil. In addition, it was shown that scuffing load increased with decreasing traction coefficient.
“Influence of Molecular Structure on the Lubrication Properties of Four Different Esters”, Tribologia, Vol. 19 No. 4, 2000, p. 3-8, compared the lubricating properties of esters. The lubrication properties that were expected to be dependent on chemical structure such as film thickness and traction, viscosity and friction coefficients were compared by experiment. The results showed that molecular length has a significant influence on lubrication properties, with longer molecules giving the highest viscosity and greatest film thickness. The length of the molecule did not influence the coefficients of friction, but the traction coefficient, gamma, decreased with increasing molecular length.
Other references of interest include U.S. Pat. Nos. 4,956,122; 4,912,272; 4,990,711; 5,858,934; and EP 088453.
The present inventors have discovered that certain fluids act as traction reducers when combined with higher viscosity fluids and that blends of traction reducers and higher viscosity fluids will increase the efficiency of gear systems.